The study — the largest one done to date on the subject — analyzed medical and dog-ownership data collected from more than 3.4 million Swedish adults, aged 40 to 80, over a 12-year period. (Since 2001, Sweden has required all dog owners to report their pets to a national registry.)

Slightly more than 13 percent of the participants owned a dog at some time during the study’s 12-year period. None of the participants had a history of cardiovascular disease at the start of the study.

Cardiovascular disease includes different diseases of the heart and blood vessels, including coronary heart disease, stroke, high blood pressure and angina (chest pain).

The analysis revealed that dog ownership in multiperson households was associated with a 15 percent reduction in death from cardiovascular disease and an 11 percent reduction in death from other fatal conditions.

Interestingly, however, dog ownership in these households had no effect on the likelihood of dying from one type of cardiovascular disease: heart attacks.

For people who lived alone, owning a dog was associated with even greater health benefits. That group experienced a 36 percent reduction in the risk of dying from heart disease and a 33 percent drop in the risk of dying from all causes.

And their risk of dying from a heart attack was also lower — by 11 percent — compared to single people who were dog-less.

The results held even after adjusting for education and income levels.

The researchers also looked at the effects of different types of dogs on their owners’ health. They found that hunting breeds, such as terriers, pointers and retrievers, were associated with the lowest risk of cardiovascular disease.

Possible explanations

What is it about owning a dog that might boost heart health? Well, one obvious explanation is that dog owners get more physical activity than non-dog owners because they have to regularly walk their pets. That may also explain why the health benefits are greater for single owners, who have no one with whom to share the daily dog-walking task. And it may explain why the benefits were greater for the owners of hunting dogs, which tend to require more exercise than other breeds.

Owning a dog (any kind) may also encourage people to work harder at rehabilitation after surgery or a serious illness because they need to get mobile to walk their pets, the authors of the study point out.

Previous research has also reported that dogs may reduce the risk of cardiovascular disease by providing emotional support and alleviating feelings of social isolation, particularly among single people and the elderly. Such factors are associated with reduced stress and lower blood pressure.

In addition, some research suggests that having a dog in the home changes the microorganisms that live on or in humans in ways that improve health — the so-called hygiene hypothesis. An earlier study by the same group of Swedish researchers found that children in households with dogs were 15 percent less likely to develop asthma than other children.

Plenty of caveats

Now, this study doesn’t prove that owning a dog gives a boost to heart health.

“These kind of epidemiological studies look for associations in large populations, but do not provide answers on whether and how dogs could protect from cardiovascular disease,” said Tove Fall, the study’s senior author and an epidemiologist at Uppsala University.

“There might also be differences between owners and non-owners already before buying a dog, which could have influenced our results, such as those people choosing to get a dog tending to be more active and of better health,” she added.

Still, the evidence is apparently strong enough for the American Heart Association to conclude (cautiously) in 2013 that dog ownership is “probably associated” with a decreased risk of cardiovascular disease and that it may even play a causal role in reducing that risk.

No one should adopt a dog simply to reduce their risk of cardiovascular disease, of course. But if you already own a dog, you might want to give him or her a little thank-you dog-biscuit treat at the end of your evening walk today.